Solar energy is the most promising source of renewable, clean energy to replace the current reliance on fossil fuels. To make solar energy viable, inexpensive materials that efficiently convert solar radiation into electricity must be developed. Ferroelectric (FE) materials have recently attracted increased attention as a candidate class of materials for use in photovoltaic devices. Their strong inversion symmetry breaking due to spontaneous polarization allows for excited carrier separation by the bulk of the material and voltages higher than the band gap due to the anomalous bulk photovoltaic effect. However, the common ABO3 perovskite FE exhibit poor carrier mobilities and wide band gaps. While a thin film configuration can dramatically increase the current harvested from the FE absorber material, the wide band gap (Egap=3-4 eV) allows the use of only 8 percent of the solar spectrum, drastically reducing the upper limit of photovoltaic efficiency. (FIG. 1)
The wide band gap of these materials is due to the fundamental characteristics of bonding in FE perovskites. The excitation across the band gap is essentially a charge transfer from the O 2p states at the valence band maximum (VBM) to the transition metal d states at the conduction band minimum (CBM). Due to a large electronegativity difference between the O and transition metal atoms, the band gap is quite large. However, it is the presence of the transition metal cations in the O6 cage that gives rise to ferroelectricity. Until now, the lowest known Egap for a FE perovskite has been 2.5 eV for BiFeO3 (BFO). This made BFO a subject of a number of investigations for photovoltaic applications, with promising results. However, even BFO is capable of absorbing only 20% of the solar spectrum. Thus, a new approach is necessary to circumvent the fundamental limitation on the Egap in FE perovskites. Accordingly, new materials are needed to be discovered and/or designed to accomplish these goals.